Abrasion-resistant laminate

ABSTRACT

An abrasion-resistant laminate is prepared by providing an ultra thin coating of micro-crystalline cellulose overcoated with an ultra thin layer of mineral particles and micro crystalline cellulose on the surface of conventional printed paper, followed by impregnating the paper with a conventional laminating resin, and then using the print paper so obtained in a laminating process without the necessity of using an overlay sheet.

FIELD OF INVENTION

The present invention relates to laminates and, more particularly,decorative laminates of high abrasion resistance. This is a division ofco-pending parent application Ser. No. 136,581, filed Apr. 1, 1980, U.S.Pat. No. 4,327,141, itself a division of application Ser. No. 966,921,filed Dec. 6, 1978, now U.S. Pat. No. 4,263,081, which is, in turn, acontinuation of Ser. No. 959,404 of Nov. 9, 1978, abandoned, and acontinuation-in-part of application Ser. No. 879,848, filed Feb. 22,1978, now U.S. Pat. No. 4,255,480, itself a continuation-in-part ofapplication Ser. No. 758,265, filed Jan. 10, 1977, abandoned. Thecontents of application Ser. No. 879,848 are incorporated herein byreference.

BACKGROUND

High pressure decorative laminates are conventionally produced bystacking and curing under heat and pressure a plurality of layers ofpaper impregnated with various synthetic thermosetting resins. In normalpractice the assembly, from the bottom up, consists of a plurality, e.g.three to eight, core sheets made from phenolic resin impregnated kraftpaper, above which lies a pattern or print sheet impregnated withmelamine resin; on top of the print sheet is provided an overlay sheetwhich, in the laminate, is almost transparent and provides protectionfor the pattern sheet.

The core sheets are conventionally made from kraft paper of about 90-125pound ream weight. Prior to stacking, the kraft paper is impregnatedwith a water-alcohol solution of phenol-formaldehyde resole, dried andpartially cured in a hot air oven, and finally cut into sheets. Theprint sheet is a high quality, 50-125 ream weight, pigment filled, alphacellulose paper that has been impregnated with a water-alcohol solutionof melamine-formaldehyde resin, dried and partially cured, and finallycut into sheets. The print sheet, prior to impregnation with the resin,usually has been printed with a decorative design, or with aphotogravure reproduction of natural materials, such as wood, marble,leather, etc.

The overlay sheet is almost invariably used when the print or patternsheet has a surface printing in order to protect the printing fromabrasive wear. The overlay sheet is a high quality alpha cellulose paperof about 20-30 pounds ream weight that is also impregnated withmelamine-formaldehyde resin in a manner similar to that used for theprint sheet, except that a greater amount of resin per unit weight ofpaper is used. The individual sheets are stacked in the manner indicatedabove and, if six sheets of impregnated core paper are used, afterlamination under heat and pressure there results a finished laminatehaving a thickness of about 50 mils, it being understood that adifferent number of sheets can be used to provide thicker or thinnerlaminates.

The stack of sheets as described above is placed between polished steelplates and subjected to about 230°-340° F. (e.g. 300° F.) at 800-1600p.s.i. (e.g. 1000 p.s.i.) for a time sufficient to consolidate thelaminate and cure the resins (e.g. about twenty-five minutes). Thiscauses the resin in the paper sheets to flow, cure and consolidate thesheets into a unitary laminated mass referred to in the art as adecorative high-pressure laminate. In actual practice, two laminatedstacks are often pressed back to back, separated by a coated releasesheet that allows the two laminates to be peeled apart after pressing.Also, a large proportion of the stacks are laminated with an aluminumfoil-kraft paper composite sheet inserted between the overlay and themetal plate, with the aluminum facing the overlay, in order to obtain alaminate having a lower gloss and a slightly textured surface which isdesirable for some products.

At the completion of the laminating operation, the backs of thelaminates are sanded to permit gluing to particle board, plywood orother substrates. The glued, laminate surfaced panel is then fabricatedinto furniture, kitchen counter tops, table tops, store fixtures andother end-use applications widely accepted for the combination ofappearance, durability and economy.

A number of variations of the above-described general process are known,particularly those operations designed to obtain special effects inappearance and texture. Also other curing cycles are possible and, infact, sometimes other resin systems are used as well.

Besides decorative high-pressure laminates referred to above, there arealso a number of low-pressure products which have been developed inrecent years, including low-pressure laminates using either saturatedpolyester resins, or melamine-formaldehyde resin. One of the fastestgrowing materials competing with high-pressure laminates in recent yearsis a product referred to as low-pressure melamine board which isnormally pressed in a short cycle at 175-225 p.s.i. at 325°-350° F.These low-pressure products have the advantage of being normally lessexpensive, but they cannot be given the title of "high pressurelaminates" because in order to be entitled to that designation, aproduct must meet a variety of rigid standards promulgated by theNational Electric Manufacturers Association, NEMA LD3-1975 whichincludes standards relating to abrasive wear, stain resistance, heatresistance, impact resistance, dimensional stability, etc. While variousother decorative printed, surfacing materials, such as some of thelow-pressure laminates, have certain of the desirable characteristics,no products other than high-pressure laminates currently available haveall of these properties.

One of these properties in particular which is very important isabrasion resistance. A high-pressure decorative laminate must havesufficient abrasion resistance to permit use in high exposure areas suchas dinette surface tops, check-out counters, etc. The standard NEMA testfor abrasion resistance is NEMA test LD-3.01. In this test a laminatesample is clamped on a rotating disc, over which ride two weightedrubber wheels, faced with calibrated sand-paper strips. As the laminatesurface is rotated under the wheels, the abrasive action of thesand-paper cuts through the surface of the laminate and graduallythrough the overlay until the printed pattern is exposed and destroyed.The NEMA standard for TYPE I laminate requires that the laminate, afterfour hundred rotation cycles, has no more than 50% of its patterndestroyed. The 50% end point is estimated by averaging the number ofcycles at which the pattern shows initial wear, and the number of cyclesat which the pattern is completely destroyed.

If a high-pressure decorative laminate is prepared in a conventionalmanner, with a normal 35-40% resin content in the print or patternsheet, but without an overlay sheet, the abrasion resistance will beonly about 50-75 cycles. If specially formulated melamine resins areused in the pattern sheet with a resin content of 50-55%, abrasionresistance of up to about 150-200 cycles are on occasion obtainablewithout an overlay sheet, but in this latter case the laminates have atendency to develop surface craze and, furthermore, they are quitedifficult to prepare due to the difficulty of impregnating the printsheet in a uniform manner; additionally, they do not meet the 400 cycleminimum required by the NEMA standard.

Nevertheless, it is desirable to produce a laminate without an overlaysheet which is capable of attaining the performance chatacteristics of alaminate using an overlay, and, in particular, one that provides a 400cycle abrasion resistance. Furthermore, it is desirable to provide alaminate which, in addition to having the 400 cycle abrasion resistance,has an initial wear point at least equal to the initial wear point of aconventional high-pressure laminate having overlay, typically 175-200cycles. This is desirable because in actual use the laminate appearancebecomes unsatisfactory not when 50% of the pattern is destroyed, butwhen a much lower percentage is destroyed. It is well known from manyyears of field experience that conventional laminates with overlay,which have 175-200 cycle initial wear point, when used in hard useareas, will have a satisfactory appearance, at least as long as thenormal replacement cycle, it being understood that replacement of mostlaminates in commercial uses is made for style reasons rather thanbecause of pattern wear. Therefore, a laminate without overlay shouldmeet these same criteria, namely it should have both a NEMA abrasionresistance of at least 400 cycles and an initial wear point in the sametest of at least 175-200 cycles, even though the latter requirement isnot part of the NEMA standard.

It is desirable to be able to provide these characteristics, but withoutusing an overlay, for several reasons:

1. Overlay adds substantial raw material costs to the manufacture oflaminates, both the cost of the overlay paper itself, the cost of theresin used to impregnate the overlay paper and the in-process andhandling losses of these materials.

2. The overlay, by imposing an intermediate layer of substantialthickness between the print sheet and the eyes of the viewer, detractssignificantly from the desired visual clarity of the pattern. Thecellulose fibers used to make overlay paper have a refractive indexclose to that of cured melamine-formaldehyde resin. The fibers aretherefore almost invisible in the cured laminate, and permit the printedpattern to be seen with very little attenuation. However, modernprinting techniques are making available very accurate reproductions ofnatural materials, particularly various wood veneer species. As theseprinted reproductions approach in appearance the natural veneer, evensmall amounts of haze or blur introduced by the overlay paper aredisturbing visually and destroy must of the realism desired by the user.

3. Furthermore, the overlay contributes to the rejection rate of thelaminate products produced. The impregnated, dry overlay sheet tends toattract small dirt particles because it develops static electricitycharges during drying. This dirt is hard to detect and remove beforelaminating, and results in spoiled laminate sheets that cannot bereprocessed. In addition, the impregnated dried overlay is brittle andhard to handle without breakage. Broken pieces are accidentally trappedon the surface of the overlay and also result in visually defectivesheets.

Additionally, overlay containing laminate, particularly those having arelatively high surface gloss, have a tendency to become dull veryquickly when subjected even to only moderate abrasive wear. This isunderstandably unacceptable where glossy laminates are desired.

The problem of providing improved abrasion resistance has been a longstanding problem in the field. Many solutions to the problem have beensuggested and, in fact, some of these have reached commercialdevelopment. Nevertheless, prior to the embodiments of the parentapplications, it has not been possible to provide a laminate, without anoverlay sheet, but having a NEMA abrasion resistance of at least 400cycles and an initial wear point in the same test of at least 175-200cycles.

It is well known that small, hard mineral particles dispersed in overlaypaper, or in resin mixtures to coat the impregnated pattern sheet, canenhance the abrasion resistance of high-pressure laminates (see, forexample, the patents to Michl, U.S. Pat. No. 3,135,643; Fuerst, U.S.Pat. No. 3,373,071 and Fuerst, U.S. Pat. No. 3,373,070). Techniques suchas these do not eliminate the overlay, but either enhance its abrasionresistance, or provide an alternate form of overlay and associatedresin.

For example, in the Barna U.S. Pat. No. 3,123,515, the overlay sheet isimpregnated with a finely divided frit, the impregnated sheet containingbetween 20 and 60% by weight of resin and frit in which the proportionof frit is between about 35 and 60% of the total solids added. Theoverlay is used in the normal manner by placing it over the print orpattern sheet.

In the Fuerst U.S. Pat. No. 3,373,070, a process is disclosed wherebysilica is incorporated into the overlay structure during the manufactureof the overlay paper itself, thereby providing a uniform distribution ofthe silica throughout the overlay sheet. This patent includes adiscussion near the bottom of column 1 of the disadvantages of the Barnatype procedure of impregnating the overlay, Fuerst being of the opinionthat a silica rich resinous coating on the top of the overlay isundesirable.

The Michl U.S. Pat. No. 3,135,642 in essence shows the casting of, orthe in situ manufacture of, an overlay sheet over the print sheet. Thecoating includes silica, finely divided cellulose flock; carboxy methylcellulose and melamine resin solids. The weight of the dry coating issaid to be 0.022 to 0.033 pounds per square foot of print sheet on thedry basis. This weight is equivalent to 66-99 pounds per ream,corresponding almost exactly to the weight range of conventionalimpregnated overlay papers, and has a thickness of about 2.5 mils (seeTable D of Michl). At best the Michl procedure provides only a minor rawmaterial cost advantage compared with the use of conventional overlay,and does not solve the problem of impaired visual effects due to haze orblur.

The Fuerst U.S. Pat. No. 3,373,071 is very similar to the Michl patent,except that the overlay cast in situ over the print sheet contains microcrystalline cellulose. This coating is said to be applied, on a dryweight basis, of 0.022 to 0.33 pounds per square foot, again giving athick coating which weighs at least 66 pounds per ream, the same minimumweight as the conventional impregnated overlay paper. Alumina insignificant amounts cannot be used in place of silica because theresultant product contains so much alumina that the products cannot becut without excessive tool wear. Even the silica, far less abrasive thanalumina, presents tool wear problems in the Fuerst products when used insignificant amounts.

One interesting technique which was briefly tested at commercial scale,but has now been abandoned, is that disclosed in the Lane et al. U.S.Pat. No. 3,798,111 in which there is disclosed the use of small mineralparticles, preferably alumina, which are incorporated within and nearthe upper layer of the base paper during its manufacture. Thus, theabrasive-resistant particles are incorporated in the paper during thepaper-making process as in Fuerst U.S. Pat. No. 3,373,070, but, moreanalogously to Barna, they are incorporated after the base layer ofpaper has been formed and is still in a wet state supported on theforming wire.

After its manufacture, this paper of the Lane et al patent issubsequently printed, impregnated and then used in the laminatingoperation as the print sheet without the necessity of using an overlay.In this process, the printing occurs above or on top of the hard mineralparticles and, consequently, high-pressure laminates produced using aprint sheet made in accordance with the Lane patent, and without anoverlay, have unacceptably low initial wear, even though they do have aNEMA abrasion resistance of at least 400 cycles. In tests, it has beenshown that laminates made with the print paper of Lane et al, withoutoverlay, had initial wear values of under 100 cycles, some as low as 35cycles. Furthermore, in a rubbing test to determine initial wear, suchlaminates began to show pattern destruction after only 3,000 rub cycles,far less than necessary.

Even if the Lane et al paper is used as an overlay, the three problemscaused by overlay and mentioned above still exist, although abrasionresistance is excellent.

Other prior art patents of some interest with regard to the backgroundof the present invention are the patents to Fuerst, U.S. Pat. No.3,445,327; Gibbons, U.S. Pat. No. 3,928,706 which suggests the use of acast in situ overlay used together with a conventional overlay, andMerriam, U.S. Pat. No. 3,661,673. Of somewhat less interest are theBattista, U.S. Pat. Nos. 3,259,537 and 3,157,518; Ando et al., U.S. Pat.No. 3,716,440; Power et al, U.S. Pat. No. 3,946,137 and Boenig, U.S.Pat. No. 3,318,760.

There are many end uses of laminates in which initial pattern wearrather than NEMA wear value determine the acceptable life of thesurface. For example, supermarket check-out counters, food servicecounters, cafeteria tables, and other commercial surfaces are exposed toabrasive rubbing and sliding of unglazed dinnerwear, canned goods,fiberglas trays, etc. If small areas of the pattern begin to disappearafter a relatively short period of use, particularly in an irregularpattern, the surface will be unacceptable to the owner and will resultin an expensive replacement. If the surface wears gradually and evenlyover a long period of time, the wear out time exceeds the normalreplacement cycle due to style changes, approximately 3-5 years.

Conventional high pressure laminates (see FIG. 1) with initial wearvalues of 175-200 are known to be satisfactory in commercial orinstitutional service, and show perhaps 10-20% pattern loss in 3-5 yearson checkout counters. To determine a predicted wear-out time forlaminate (FIG. 2) without overlay, made using the print paper of theLane et al. U.S. Pat. No. 3,798,111, such laminates along withconventional laminates and those made in accordance with embodiments ofthe parent applications were subjected to an abrasive rub testconsisting of sliding a simulated fiberglas tray surface back-and-forthover the test laminate, the simulated fiberglas tray surface beingbonded to the bottom of a No. 10 can carrying 5 lbs. of weight, andflexibly clamped in a cam driven jig that provided about 5 inches ofoscillatory motion. In this test, the laminate according to U.S. Pat.No. 3,798,111 began to show pattern destruction after about 3000 rubcycles. Conventional laminate with overlay and laminate prepared inaccordance with embodiments of the parent applications without overlaydid not show any pattern destruction after 30,000 cycles.

The "rub test" or "sliding can test" was also used to compare theembodiments of the parent applications with conventional mirror-surfacedlaminates having overlay. As previously noted, both start initialpattern destruction at about 30,000 rub cycles. The conventionallaminate shows gradual surface dulling beginning almost with the firstfew hundred rub cycles, and is completely dulled well before initialpattern destruction. The abrasion-resistant embodiments of the parentapplications, however, showed negligible surface dulling almost up tothe point of pattern destruction. These results suggest not only animportant advantage of these laminates compared with conventionallaminates including overlay, but also similar advantages compared withlaminates produced by the casting of the overlay in situ on the printsheet, e.g. the Fuerst U.S. Pat. No. 3,373,071.

Even after the considerable activity in the field in order to solve theproblems indicated above, these problems have not been solved until theembodiments of the parent applications. In the parent applications thetechnique exemplified utilizes a single ultra-thin layer comprising amixture of binder and abrasion-resistant particles. While the laminatesso produced are far superior to all prior attempts, it has now beenfound that in a minority of patterns, i.e. heavily inked patterns, andsome printed on smooth paper, it is necessary--to achieve good initialwear--to provide the single ultra-thin layer in undesirably heavythicknesses, i.e. as much as 8-12 lbs/ream or even as high as 16lbs/ream, whereas most patterns are provided with good initial wear inaccordance with the parent applications with ultra-thin coatings of only4-5 lbs/ream.

While the heavier coatings applied to the heavily inked patterns, stillultra-thin in comparison with the prior art, provide superior initialwear and sliding can values, these laminates (having abrasive-resistantcoatings much above 6 lbs/ream where the abrasive particles are alumina)are difficult to handle in the sense that tools used to cut suchlaminates are quickly worn out by the quantities of alumina present, andchipping at a rapid rate during machine routing operations sometimesoccurs.

SUMMARY

It is, accordingly, an object of the present invention to overcome thedeficiencies of the prior art and also of the embodiments of the parentapplications, such as indicated above.

It is another object of the present invention to provide for an improveddecorative laminate.

Another object of the invention is to provide a high-pressure decorativelaminate that does not contain an overlay sheet, but which neverthelessmeets the requirements of the NEMA abrasion standard, and furthermoreprovides an initial wear point of at least 175-200 cycles in this sametest.

It is yet another object of the present invention to provide such ahigh-pressure laminate using conventional base papers used for theprinting of the pattern or print sheet.

It is still a further object of the present invention to provide aprocess whereby the printed pattern sheet is impregnated utilizingconventional impregnation and drying equipment commonly used in thelaminating field.

It is a further object of the present invention to provide for improveddecorative laminates in a manner which does not require substantial rawmaterial costs and which significantly enhance the economics of laminateproduction by the elimination of the overlay sheet.

Another object of the present invention is to provide a process formaking laminates which involves significant cost reduction and resultsin a product having improved appearance and which has the potential forpreviously unavailable additional novel graphics development.

Another object of the present invention is to provide for improvedlow-pressure laminates including the up-grading of low-pressure melamineboard.

A further object is to reduce the quantity of abrasion-resistantmaterial necessary to achieve superior abrasion resistance, withoutcausing excessive tool wear in the cutting of the product.

These and other objects of the invention are attained by coatingconventional printed or otherwise decorated pattern paper first with anultra thin coating of suitable binder and then with an ultra thincoating containing small mineral particles immobilized by preferably thesame binder, and wherein such print sheet is then impregnated in thenormal manner with a suitable thermo-setting resin such as melamineresin, and then using the print sheet in the production of decorativelaminates without an overlay sheet. The abrasion-resistant particlesare, therefore, used in lesser quantities and are more concentrated onthe uppermost surfaces of the laminates compared with examples of theparent applications giving equivalent wear resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and the nature and advantages of the instantinvention will be more apparent from the following detailed descriptionof embodiments taken in conjunction with the drawing (not to scale)wherein:

FIGS. 1-3 are schematic sectional views of laminates in accordance withthe prior art;

FIG. 4 is a laminate in accordance with embodiments of the parentapplications;

FIG. 5 is a flow-diagram showing a method of preparing a print layer inaccordance with the present invention;

FIG. 6 is a schematic sectional view, not in scale, showing anembodiment of the print sheet in accordance with the present invention;and

FIG. 7 is a schematic sectional view, not in scale, showing a laminatein accordance with the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Unless paper is highly calandered or otherwise pressed to a very smoothsurface, its surface is fairly rough. When paper is printed the inksinks partly into the upper surface of the paper and partly projectsabove its upper surface thereby making the upper surface even more roughand irregular. This is the nature of the print paper used in themanufacture of laminates. Some heavily printed patterns in particularhave rough surfaces and, accordingly, when an ultra-tinabrasion-resistant coating in accordance with embodiments of the parentapplications are applied thereto, the valleys between the peaks wherethe printing occurs are only partly filled with the result that theabrasion-resistant coating over such peaks is thinner and the surface isstill somewhat irregular as schematically illustrated in FIG. 4. Thetendency of the coating to level and provide a thinner layer over thepeaks is accentuated when the inks are composed of hydrophobicthermoplastics, as is common.

To overcome this problem various solutions are proposed:

(1) the incorporation in the printing inks used on the print paper ofsilanes to better bond the abrasion-resistant particles of the coatingthereto, and/or the incorporation of wetting agents in the inks tobetter permit wetting of the otherwise hydrophobic ink peaks;

(2) the provision of a second ultra-thin binder layer over the mainultra-thin layer of binder/abrasion-resistant particles to increase thequantity of melamine resin at the surface of the laminate; and

(3) the provision of such a second ultra-thin binder layer below themain ultra-thin abrasion-resistant layer.

While all three alternatives give improved results, the thirdalternative gives by far the most superior results. Therefor, inaccordance with the present invention, there is first applied to theprint or pattern paper an ultra-thin coating of binder, preferablymicrocrystalline cellulose of average thickness about 0.05 to 0.3 mils(dry), i.e. applied at a rate of about 1/2 to 3 pounds per ream. Thiscoating, which tends to level the roughness of the upper surface of theprint sheet and also to coat the peaks sufficiently to form a betteranchor base for the abrasion-resistant layer than does the ink peaks, isthen dried. The ultra-thin abrasion-resistant coating is then applied ata much lower rate without being wasted in the valleys compared with theembodiments of the parent applications, e.g. the abrasion-resistantlayer is applied at the rate of only 1-6 pounds per ream to provide adry upper layer thickness of 0.02-0.2 mils (usually less than 0.15 milsthick) over the first coating which serves as a substrate.

As in the parent applications, the abrasion-resistant composition,containing small mineral particles, when coated without resin overunimpregnated printed pattern paper having a sub-coating thereon,provides surprising and unexpected properties by permitting such paperto be used in the preparation of decorative laminates without an overlaysheet and wherein the resultant laminates are highly abrasion-resistant,using only small amounts of mineral particles and without causing theheavily-inked product to be unduly hard on cutting tools.

In its preferred form, the coating composition is composed of a mixtureof small particles of alumina or other abrasion-resistant particles ofaverage 20-50 micron particle size, and a lesser amount ofmicro-crystalline cellulose particles, both dispersed in a stable,aqueous slurry. The particles of alumina, of small size such that theydo not interfere with the visual effects in the final product, serve asthe abrasion resistant material and the micro-crystalline celluloseparticles serve as the preferred temporary binder. It will be understoodthat the binder must be compatible with the resin system later utilizedin the laminating procedure, usually a melamine resin or in the case ofcertain low-pressure laminates a polyester resin system, and themicro-crystalline cellulose serves this function as well as stabilizingthe small particles of alumina on the surface of the print sheet.

The sub-layer is also formed of a temporary binder having the sameproperties as the binder of the abrasion-resistant layer. Preferably thebinder is the same binder, most preferably micro-crystalline cellulose.

With reference to FIG. 5, in the preferred operation a conventionalunimpregnated print or pattern paper is coated first with the bindermaterial at a rate of about 1/2 to 3, preferably about 11/2 to 21/2,lbs./ream and the coating is leveled and dried. As seen schematically inFIG. 6, the binder layer tends to level and fill in the holes in thesurface, e.g. between the ink peaks while providing a coating thereoveras well; therefore, while the average thickness of the binder layer maybe as great as 0.3 mil, its thickness over the peaks will always be muchless, normally less than 0.1 mil. The dried layer is then coated withthe mixture of hard mineral particles and binder, preferably alumina andmicro-crystalline cellulose particles in a stable aqueous slurry,normally at a rate of about 1-6 lbs. per ream, and the coating is driedat an elevated temperature of at least 140° F. and preferably 180° F.,such as in a hot-air oven, to produce a thin over-coating only 0.02 to0.2 mils thick, usually less than 0.15 mils thick.

The resultant abrasion-resistant coated paper (FIG. 6) is thenimpregnated with the melamine or polyester resin and dried in aconventional way, at which point it is ready for the laminatingprocedure. Based on the comparative weights of the print sheet and themicro-crystalline cellulose in the ultra-thin coatings thereon, and thetotal amount of melamine resin impregnated thereinto, it is calculatedthat only 2-13 parts of micro-crystalline cellulose are used per 100parts of resin.

With reference to FIG. 7, it is seen that the abrasion-resistant resinimpregnated print sheet, having an ultra-thin abrasive-resistant coatingon its upper surface, is assembled for the laminating step in theconventional way, except that no overlay sheet is used. The laminate isthen cured under heat and pressure in the conventional manner. Asurprising characteristic of the ultra-thin coating is that even thoughits total thickness (both layers) is so thin after pressing it canprovide abrasion resistance in the finished laminate not only meeting400 cycles NEMA Standard, but also providing an initial wear point inexcess of 175-200 cycles.

It is also surprising that the coatings forming the total ultra-thinlayer tightly adhere to the surface of the printed paper when the paperis later impregnated with melamine resin, without significant amounts ofthe mineral particles either being lost in the impregnating solution ormigrating away from the surface of the paper. A further surprisingcharacteristic of these coatings is that they do not appear to hinderthe penetration of the melamine-formaldehyde resin solution into theinterior of the paper, during the impregnation step; such penetration isessential, or the pattern sheet will be irregularly starved such as atits center, and could possibly delaminate after pressing. A furtherdesirable characteristic of the coatings is that they do notsignificantly scatter or attenuate light, resulting in very clear, crispappearance of the pattern in the finished laminate. Also, because of thevery ultra-thin nature of the mineral particle containing layer, thereis no tool wear problem during the subsequent processing of theresultant laminates.

Without being bound to the following theory, it is believed that theimproved characteristics of the invention can be accounted for asfollows. Microcrystalline cellulose particles contain very largeexternal forces that bind to other polar substances, such as celluloseand alumina. Thus, an aqueous slurry of microcrystalline cellulose andalumina is stable and does not quickly settle out, even though aluminaparticles in water are not stable. Furthermore, when this slurry iscoated onto the paper or the subultra-thin binder layer, themicrocrystalline cellulose apparently binds the alumina particles to thesurface fibers of the paper or to the binder sub-layer, preventingmigration of the alumina particles to below the surface. This mayaccount for the good abrasion resistance developed by such very smallquantities of alumina. Thus, all or substantially all of the aluminaparticles stay at the surface where they do the most good, rather thanbecoming dispersed below the surface where they would contributerelatively little initial wear resistance.

As indicated above, the preferred slurry composition for theabrasion-resistant ultra-thin upper layer contains a mixture of smallparticles of alumina and a lesser amount of microcrystalline celluloseparticles, both dispersed in water. There must be an amount sufficientof the small mineral particles to provide the resultant produce with thedesired abrasion resistance as discussed above, and there must be anamount sufficient of the binder to retain the mineral particles in placeon the surface of the ultra-thin-sub-layer.

In general, it has been found that satisfactory results are attainedwith at least about 5 parts e.g. about 5 to 10 parts, by weight of themicrocrystalline cellulose for about 20-120 parts by weight of thealumina; it is possible to work outside this range. The quantity ofwater in the slurry is also dictated by practical considerations, sinceif there is too little water the slurry becomes so thick that it is hardto apply; similarly, if there is too much water the slurry becomes sothin that it is difficult to maintain a consistent thickness during thecoating operation due to running of the slurry. Thus, a slurrycontaining about 2.5 wt % microcrystalline cellulose and about 28 wt %alumina, based on the water, is stable, i.e. the alumina does not settleout; but if more than about 3.5 wt % micro-crystalline cellulose andabout 28 wt % alumina, based on the water, is used, the slurry becomesvery thixotropic and difficult to apply. A preferred slurry containsabout 3.2% microcrystalline cellulose and about 9% alumina.

The composition also preferably contains a small amount of wettingagent, preferably a non-ionic wetting agent, a small amount of asubsidiary suspending-binding agent such as carboxy-methyl cellulose,and a small amount of silane. The quantity of wetting agent is notcritical, but only a very small amount is desirable and excessquantities provide no advantage. If a subsidiary suspending-bindingagent, such as carboxy methyl cellulose is used to help keep the mineralparticles in suspension, it may be used in amounts up to about 50% byweight of the amount of the microcrystalline cellulose.

If a silane is used, it acts as a coupling agent* which chemically bindsthe alumina or other inorganic particles to the melamine matrix afterimpregnation and cure, and this provides better initial wear since thealumina particles are chemically bound to the melamine in addition tobeing mechanically bound thereto and therefore stay in place longerunder abrasive water. The silane should be selected from among the groupmaking it compatible with the particular thermosetting laminating resinused; in this regard silanes having an amino group, such asgamma-aminopropyl trimethoxy silane (Silane A-1110) or gamma-aminopropyltriethoxy silane (Silane A-1110) are particularly effective for use withmelamine resins. The quantity of silane used need not be great and, infact, as little as 0.5% based on the weight of the alumina is effectiveto enhance the abrasion resistance of the final laminate; a maximumquantity of about 2% by weight based on the weight of the alumina orother hard particles is suggested since greater quantities do not leadto any significantly better results and merely increase the cost of theraw materials.

It is an important feature of the present invention that the coatingusing micro-crystalline cellulose as the binder must be dried at anelevated temperature before the print sheet is impregnated with themelamine resin. Thus, a minimum drying temperature is about 140° F. webtemperature and the preferred drying temperatures are from 160°-180° F.web temperature, or even higher.

With regard to the abrasion-resistant mineral particles, alumina is thepreferred material. Silica, which has been suggested in certain priorart patents as an abrasion-resistant material, provides considerablyinferior results in the present invention compared with alumina, but canbe used. Other minerals of sufficient hardness such as zirconium oxide,cerium oxide, diamond dust, etc. can work, but are either too expensivefor practical usage or under certain circumstances produce excessivecolor shift. Glass beads have been tried unsuccessfully. Silicon carbidealso was tried, and while providing good abrasion resistance, producedexcessive color shift. Mixtures of silica and alumina give good results.

An important feature is the size of the alumina or other hard particles.Beneath 20 micron particle size, abrasion resistance becomes poor, andthe preferred minimum average particle size is about 25 microns. Maximumaverage particle size is limited by surface roughness in the article andinterference with visual effects. The preferred maximum average size ofthe abrasive resistant particles is about 50 microns.

The nature of the binder for the mineral particles is a very importantfeature in the present invention. Of all the materials tried,microcrystalline cellulose is by far the most satisfactory material. Thebinder must serve not only to maintain the mineral particles in positionon the surface of the sub-layer on the print sheet, but should also actas suspending agent in the slurry (otherwise, it would be necessary toadd an additional suspending agent). The peculiar property ofmicrocrystalline cellulose is that it acts like a typical suspendingbinding agent and film former, but unlike other agents is not watersoluble before or after suspension and forms a highly porous filmthrough which the thermosetting resin can penetrate. In addition, thebinder must be compatible with the laminating resin and microcrystallinecellulose is compatible with both melamine resin and polyester resins.Furthermore, it must not scatter or attenuate light in the thicknessesapplied in the final laminate, and microcrstalline cellulose issatisfactory in this regard as well.

Other binders which may be used, but which provide inferior resultscompared with microcrystalline cellulose, are various typicalsuspending-binding agents including anionic acrylic polymer, carboxymethylcellulose and similar materials such as hydroxypropyl cellulose,methylcellulose, polyvinyl alcohol, polyvinyl pyrrolidone, etc. However,as indicated above, microcrystalline cellulose is by far the preferredbinder.

Microcrystalline cellulose is a non-fibrous form of cellulose in whichthe cell walls of cellulose fibers have been broken into fragmentsranging in length from a few microns to a few tenths of a micron. It isnot a chemical derivative but a purified alpha cellulose.Microcrystalline cellulose is available under the trademark "AVICEL",the preparation of which is disclosed in the Battista U.S. Pat. No.3,275,580. AVICEL Type RC 581 is a white, odorless hygroscopic powder.It is water dispersible and contains about 11% sodium carboxymethylcellulose as a protective colloid. Its particle size is less than 0.1%on a 60 mesh screen.

The ultra-thin sub-layer is much like the ultra-thin abrasion-resistanttop layer, except that it preferably contains no abrasion-resistantparticles. An aqueous slurry containing about 3-6 wt % microcrystallinecellulose can be satisfactorily coated over the print layer to form asubstrate for the ultra-thin abrasion-resistant layer. A small amount ofwetting agent may be used.

Features and advantages of the instant invention which are considered tobe particularly significant are as follows:

(1) The mixture of mineral particles and microcrystalline cellulose isdeposited from a water slurry, rather than used as fillers in a resinsolution. The abrasion-resistant mineral particles are thereby highlyconcentrated in the resultant layer.

(2) Such slurry is coated on an unimpregnated printed pattern sheet,rather than on an impregnated pattern sheet.

(3) The coating is dried at an elevated temperature of at least 140° F.

(4) The total average coating thickness of both layers is only 0.07 to0.5 mils after pressing, with the upper abrasion-resistant layer being amaximum of 0.2 mils thick, rather than requiring an overlay of 1-2 milsthickness.

(5) After applying the coatings and drying, the pattern sheet is thenimpregnated with the thermosetting resin, and this conventionalimpregnation of the pattern sheet is carried out on conventionalequipment, rather than special, difficult to control, coating of a thickslurry.

(6) The ultra-thin upper layer provides unexpectedly high abrasionresistance.

The desirable characteristics of the mineral particle binding agent,which characteristics are all met by microcrystalline cellulose, are: Itacts as a film former; it acts as a binding agent for the mineralparticles; it acts as a suspending agent in the slurry for the mineralparticles; it is not washed off during the subsequent thermosettingresin impregnating process; it is compatible with the subsequentlyapplied thermosetting resin, such as melamine resin or polyester resin;it is permeable to the thermosetting impregnating resin (indeedmicrocrystalline cellulose forms a porous film); it is resistant to theheat generated during the laminating procedure; and it does not scatteror attenuate light in the laminate.

The following examples are offered illustratively:

EXAMPLE I A--Top Coating

A typical composition in accordance with the parent applications wasapplied in an ultra-thin layer in accordance with said applications at acoating weight as shown in Table I to print sheets having a heavilyinked pattern. After drying, an Avicel coating was applied at 2 lbs/ream(dry) in an ultra-thin layer over the ultra-thin abrasion-resistantlayer, and the top coating was dried; the sheets were then impregnatedwith melamine resin, the top coating providing for increased melaminecoverage and the so-prepared print sheets were used to form laminateswithout any overlay sheet.

B--Under Coating

The process of EXAMPLE I-A was repeated except that the 2 lbs/ream (dry)ultra-thin Avicel coating was applied directly to the upper surface ofthe print sheets and the ultra-thin abrasion-resistant layer was appliedover the dried Avicel layer. By filling the depressions on the printsheet surface first with Avicel, the paper surface was made more leveland all of the abrasion-resistant coating was placed over the patternwhere it does the most good. No abrasion-resistant coating was wasted inthe valleys.

As a comparative example, alaminate was made in accordance with theparent application using only the ultra-thin abrasion-resistant layer.Results may be found in TABLES I and II.

                                      TABLE I                                     __________________________________________________________________________    Initial Wear Abrasion Resistance                                              HEAVILY                                                                              DRY COAT WEIGHT    A     B                                             INKED  OF ALUMINA   SINGLE                                                                              TOP   UNDER                                         PATTERNS                                                                             CONTAINING LAYER                                                                           COAT  COATED                                                                              COATED                                        (NUMBER)                                                                             (lbs/REAM)   (CYCLES)                                                                            (CYCLES)                                                                            (CYCLES)                                      __________________________________________________________________________    AN-1-1 6            100   300   --                                            AN-1-1 5.6          --    --    500+                                          W-8-104                                                                              5             50   400   --                                            W-8-104                                                                              5.6          --    --    500+                                          __________________________________________________________________________

It should be noted that the initial wear shown here for single coatedlaminate is lower than shown in examples of the parent applications.This is characteristic of certain heavily inked patterns, of which thesetwo are examples.

                                      TABLE II                                    __________________________________________________________________________    Sliding Can Test                                                                    DRY COAT WEIGHT    A     B                                                    OF ALUMINA   SINGLE                                                                              TOP   UNDER                                          PATTERN                                                                             CONTAINING LAYER                                                                           COAT  COATED                                                                              COATED                                         NUMBER                                                                              (lbs/REAM)   (CYCLES)                                                                            (CYCLES)                                                                            (CYCLES)                                       __________________________________________________________________________    AN-1-1                                                                              6            4,627  6,057                                                                              --                                             AN-1-1                                                                              5.6          --    --    40,271+                                        W-8-104                                                                             5            6,500 25,433                                                                              --                                             W-8-104                                                                             4.5          --    --    40,825+                                        __________________________________________________________________________

From this it can be seen that although top coating improves wear, basecoating is more effective.

EXAMPLE II

A sample production run of base coated paper was made on an air knifecoater. Formulations were as follows:

    ______________________________________                                        UNDER COATING                                                                 ______________________________________                                        Water                 300 lbs.                                                Avicel RC 581          10 lbs.                                                ______________________________________                                        ABRASION-RESISTANT COATING                                                    ______________________________________                                        Water                    300 lbs.                                             Avicel RC 581            11.2 lbs.                                            CMC 7L                   1.4 lbs.                                             Alumina of 30μ average diameter                                                                     30.0 lbs.                                            Silane A-1100            0.7 lbs.                                             ______________________________________                                    

A heavily printed woodgrain pattern, W-8-104, was coated with 1.4lbs/ream of Avicel under coating and dried. This was followed with 4.2lbs/ream of abrasion-resistant coating which was then also dried. The sodouble coated paper was treated with melamine-formaldehyde resin to aresin content of 42-44% and volatile of 5-6%. Laminates without overlaywere pressed using this print sheet. Initial wear on laminate made fromthis paper was 510 cycles. Sliding can test gave over 40,000 strokes. Incomparison, noting TABLE I above, single coated laminates having onlythe ultra-thin abrasion-resistant layer applied to the same heavilycoated print sheet, W-8-104, at the rate of 5 lbs/ream, had an initialwear of only 50 cycles.

Sheets of W-8-104 in 0.050" thickness were glued to 3/4" particleboardwith PVA adhesive. Laminate with a single coat, such as tested in TABLEI at 6 lbs/ream, was compared for "machineability" with the dual coat at4.2 lbs/ream over 1.4 lbs/ream of base coating. The single coatformulation contained 86.7% alumina; therefore, it contained 6lbs/ream×0.867=5.2 lbs/ream of alumina. Dual coat contained 69.2%alumina or 4.2 lbs/ream×0.692=2.9 lbs/ream of alumina.

Both panels were machined using carbide flat knives on a shaper. Singlecoat laminate started slight chipping at about 100 linear feet ofmachining. Dual coat took about 1000 linear feet to show similarchipping. From this it can be seen that dual coat at a little more than1/2 alumina weight had more than 10 times the initial water and about1/10 tool wear.

Compared with the prior attempts, the present invention provides vastlyimproved results such that the present invention can be truly consideredto be a revolutionary development in the field of decorative laminates.Insofar as is known, the present invention provides for the first time,except for the embodiments of the parent applications, a laminatewithout an overlay sheet has been made which is capable of meeting boththe NEMA Abrasion Resistance Standard of at least 400 cycles, and aninitial wear point in this same test of at least 175-200 cycles.

The closest thing previously available (see FIG. 2) has been the use ofprint paper made in accordance with the Lane et al. U.S. Pat. No.3,798,111. While laminates made using this paper, without an overlay,have excellent abrasion values according to the NEMA Standard, theinitial wear point in these products, however, is still very poor. Testsconducted on such laminates show that many have initial wear values ofunder 100 cycles, some as low as 35 cycles, whereas conventionallaminates made with conventional overly have an initial wear point of175-200 cycles. In contrast, laminates made in accordance with thepresent invention have initial wear points of no less than 175 cycles(usually a minimum of 200 cycles) and up to about 500 cycles.

In addition to providing poorer initial wear values, the laminates madewithout overlay using the print paper of the Lane et al. U.S. Pat. No.3,798,111 provide other disadvantages as well. In the Lane et al patent,the alumina particles are introduced during the paper making process andthis results in a special grade paper for each base paper colorrequired, greatly increasing inventory requirements; in contrast, in thepresent invention in which the coatings are applied after printing, useis made of all existing stocks of conventional print paper. Furthermore,the present invention is more flexible than the Lane et al. process inthat is permits tailoring of the abrasion resistance to specific needs,without the cumbersome redevelopment of a paper base on a paper machine.

The present invention is also believed to constitute other importantimprovements over the casting of an overlay sheet (Michl, U.S. Pat. No.3,135,643 and Fuerst, U.S. Pat. No. 3,373,071; and other patents). Suchpatents show what is in essence the casting in situ of a resincontaining paper layer onto resin impregnated pattern sheet, in whichthe upper resin paper layer contains abrasion-resistant mineralparticles (see FIG. 3). In these techniques, the base paper alreadycontains the melamine resin and the thick coating applied also containsmelamine resin. The coating is applied at the rate of, at the veryleast, 0.022 lbs. per sq. ft. which is a quantity on the order of about8 times or more as great as that used in the present invention. Thethickness of the dry layer runs at least 2 mil and preferably morecompared with calculated dry coatings in the present invention runningfrom 0.07 to 0.5 mils, the abrasion layer being preferably a maximum ofonly 0.15 mils.

The Michl patent essentially discloses how to deposit an overlay layeronto the impregnated pattern sheet, rather than how to eliminate theoverlay. The finished laminate (FIG. 3) is essentially the same as thatof a conventional laminate, containing cellulose fibers, resin, anddiffers only by the mineral particles dispersed in this layer. TheFuerst U.S. Pat. No. 3,373,071 states that the process taught by Michlresults in laminates that are blotchy when satinized. A significantpercentage of laminates are satinized with pumice and rotating brushesto reduce their surface gloss. The Fuerst patent replaces the cellulosefiber of the Michl process with microcrystalline cellulose in order toprovide blotch-free surfaces after satinizing. Thus, the basic processof Fuerst is the same as that of Michl, i.e. depositing a resincontaining overlay layer onto the wet, impregnated pattern sheet, thethickness of the Fuerst coating being at least four times as thick asthe maximum of those found useful in the present invention, and offeringno significant savings of raw material compared with conventional,overlay containing laminate. The most significant advantage of thepresent invention compared to such prior art is, however, the vastlyimproved abrasion resistance as evidenced by improved initial wear,improved NEMA abrasion resistance and reduced dulling during the slidingcan test. Tool wear is also a problem with the Michl and Fuerstlaminates, where large quantities of mineral are used.

It will be obvious to those skilled in the art that various changes maybe made without departing from the scope of the invention and theinvention is not to be considered limited to what is shown in thedrawings and described in the specification. For example, it will beunderstood that certain additional variations in processing will, incertain instances, give somewhat different results. For example, resultsare generally better when the laminates in accordance with the presentinvention are formed against a hard surface. Thus, plate producedfinishes, such as mirror and satin, provide better initial wear undergiven coating conditions than do foil (or other soft-backed pressingsurfaces) produced finishes. Accordingly, in some instances it isadvantageous to calender the dried, coated pattern sheet prior toimpregnation with the thermosetting resin.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without departing from the engineering conceptand, therefore, such adaptations and modifications should and areintended to be comprehended within the meaning and range of equivalentsof the disclosed embodiments. It is to be understood that thephraseology or terminology employed herein is for the purposes ofdescription and not of limitation.

What is claimed is:
 1. In a print sheet for use in the preparation ofdecorative laminates of high abrasion resistance, comprising: a papersheet substrate having a print design thereon, and an ultra-thinabrasion-resistant coating over said print design, said ultra-thinabrasion-resistant coating comprising a mixture of (1) anabrasion-resistant hard mineral of particle size 20-50 microns in highconcentration sufficient to provide an abrasion-resistant layer withoutinterfering with visibility and (2) binder material for said mineralcompatible with a thermosetting laminating resin selected from the groupconsisting of melamine-formaldehyde resin and polyester resin, saidcoating being impregnable by said laminating resin and substantiallytransparent together with said resin after curing of said resin, saidbinder material being present in an amount sufficient to bind andstabilize said abrasion-resistant mineral to the surface of said papersheet, the improvement comprisingan ultra-thin layer of binder materiallocated either directly over the surface of said print sheet and beneathsaid ultra-thin abrasion-resistant coating, or above saidabrasion-resistant coating.
 2. A print sheet in accordance with claim 1,wherein said ultra-thin layer of binder material lies directly over thesurface of said print sheet and beneath said ultra-thinabrasion-resistant coating.
 3. A print sheet in accordance with claim 2impregnated with a thermosetting laminating resin selected from thegroup consisting of polyester resin and melamine-formaldehyde resin. 4.A print sheet in accordance with claim 2, wherein said binder materialcomprises microcrystalline cellulose.
 5. A print sheet in accordancewith claim 2, wherein said abrasion-resistant mineral is alumina, silicaor mixtures thereof.
 6. A print sheet in accordance with claim 2impregnated with melamine resin, wherein said abrasion-resistant mineralis alumina, silica or mixtures thereof, and said binder ismicrocrystalline cellulose, said ultra-thin abrasion-resistant coatinghaving a calculated thickness of 0.02-0.15 mils and comprising at least5 parts by weight of said microcrystalline cellulose for about 20-120parts by weight of said mineral, and said ultra-thin binder layer havingan average calculated thickness of 0.05-0.3 mils.
 7. A print sheet inaccordance with claim 2 or 6, wherein said coating further comprises 0.5to 2.0% by weight of a silane based on the weight of said mineral.
 8. Aprint sheet in accordance with claim 1 wherein the quantity of bindermaterial in said abrasion-resistant coating is no greater than thequantity by weight of said mineral in said abrasion-resistant coating.9. In a print sheet for use in the preparation of decorative laminatesof high abrasion resistance, comprising:a paper sheet substrate having aprint design thereon, and an ultra-thin abrasion-resistant porouscoating over said print design, said ultra-thin abrasion-resistantcoating having a thickness after pressing on the order of 0.02-0.3 milsand comprising a mixture of (1) an abrasion-resistant hard mineral ofsmall particle size sufficient to provide an abrasion resistant layerwithout interfering with visibility and (2) binder material for saidmineral, said binder material being present in an amount sufficient tobind and stabilize said abrasion-resistant mineral over the substrate,the improvement comprising a porous layer of binder material locatedeither directly over the surface of said print sheet and beneath saidultra-thin abrasion-resistant coating, or above said abrasion-resistantcoating.
 10. A method of producing a decorative facing sheet for use inthe manufacture of thermoset resin impregnated abrasion resistantdecorative laminates having enhanced abrasion resistance without anoverlay layer, the method comprising:coating a decorative facing sheetwith an ultra-thin wet layer of binder material capable of withstandinglaminating conditions to make the laminate, being compatible with thethermoset resin, being in the dry state permeable to the resin before ithas become thermoset and together with the thermoset resin beingsubstantially transparent; drying said wet ultra-thin binder materiallayer to provide an ultra-thin porous dry binder material layer; coatingsaid ultra-thin porous dry binder material layer with an ultra-thin wetlayer of a mixture of (1) an abrasion-resistant hard mineral of particlesize 20-50 microns in a quantity sufficient to provide anabrasion-resistant layer without interfering with visibility and (2)binder material for said mineral which binder material is capable ofwithstanding the subsequent laminating conditions, is compatible withthe resin before it has become thermoset, together with said thermosetresin is substantially transparent, said binder material being presentin an amount sufficient to bind said abrasion-resistant mineral to thesurface of said ultra-thin porous dry binder material layer; drying saidmineral-binder material mixture on said ultra-thin binder material layerat a temperature sufficient to enhance the bonding of saidabrasion-resistant mineral by said binder material to the surface ofsaid ultra-thin porous dry binder material layer, to provide anultra-thin porous dry layer of said mixture thereon.
 11. A method inaccordance with claim 10 further comprising impregnating said coatedfacing sheet with a thermosetting resin.
 12. A method in accordance withclaim 10 wherein said binder comprises microcrystalline cellulose andthe drying thereof is effected at a temperature of at least 140° F.